1. Field of the Invention
The invention relates to a gasification device for the introduction of air into wastewater.
2. The Prior Art
A gasification device of this generic type is known from German Patent No. PS 38 19 305. An introduction body element of this gasification device is made of a plastic and the membrane of an elastomer. The membrane contains perforation slits through which the gas can pass into the waste water which is to be treated. It can also pass into areas without perforation slits, e.g., directly at a gas distribution slot, to prevent an indirect and excessively concentrated emergence of gas at that point.
The basic design of this gasification device has been tried and tested. With regard to the selection of the material for the membrane, considerable progress has been made recently, which has largely succeeded in reducing deposits of slime and encrustations by organic and inorganic substances as well as losses of softener. The length of service life which is possible with this progress is limited by the fact that mechanical damage can occur to the membrane due to cracks which did not previously occur due to the earlier replacement required of the membrane.
It is an object of the present invention to extend the service life of the membrane of a gasification device by reducing the propensity to crack formation.
This and other objectives are achieved by providing a gasification device for the introduction of air into wastewater having a blower body element, a membrane surrounding the blower body element and a longitudinal area of said membrane that forces the creation of an axial fold of said membrane in an upper area.
The solution of the invention required extensive investigations into the causes of cracks, which was difficult because a gasification device cannot be monitored in practical operation. The following was discovered:
Crack formation occurs if the gasification device is not operated continuously but is intermittently operated. The reason for this intermittent gas introduction is the treatment in clarification plants for a number of types of waste water. This is based initially on exploiting the ability of bacteria, predominantly aerobic bacteria, to aerate organic substances, i.e., to break them down in the final analysis into carbon dioxide, water, nitrates and final analysis into carbon dioxide, water, nitrates and sulphates. A precondition for this in aerobic operating systems is an adequate aeration of the activated sludge in the waste water. The aeration is then stopped and the biological waste water purification with anaerobic agents, i.e., bacteria which live off chemically-bonded oxygen and therefore carry out reductive decomposition, increases in importance.
Due to intermittent operation, pauses arise in which no gas is introduced into the waste water, and the membrane is then no longer inflated by a gas cushion between its inner side and the outer side of the introduction body element. The inflation forms a fold in the upper part of the membrane. The research into the causes identified the following reasons for this:
When the aeration process is switched off, the hydrostatic pressure in the waste water of the clarification plant has the effect of pressing the membrane against the introduction body element. However, because this does not always take place uniformly it is possible that an uncontrolled fold formation takes place, which runs into the area of the perforation slits, and thereby gradually leads to the fatigue of the material.
Specifically, the area of the membrane with the perforation slits is less stiff than the remaining unperforated area. Therefore, it tends to initiate uncontrolled fold formation in the perforated area. Once a fold has started in that area, it continues to develop. However, the perforation slits already represent a weakness in the material, and facilitate tearing of the membrane as the fatigue of the material progresses.
The effect of fold formation is further facilitated if the membrane is xe2x80x9coversizexe2x80x9d; i.e., its circumference in the relaxed state is greater than the circumference of the introduction body element. This is frequently the case in practice, since an oversize is already present at manufacture facilitates fitting and the membrane can be made taut without resistance over the introduction body element. In addition to this, a sustained expansion of the membrane occurs with time during operation due to the imposition of pressure from within.
Within the gasification device according to the invention, uncontrolled fold formation is avoided, in that a controlled fold formation is forced into effect. In this situation, the controlled fold formation is effected in an area of the membrane which is free of perforation slits and therefore not already weakened. Once a fold occurs, the xe2x80x9cexcessxe2x80x9d material which is present can dam up the circumference of the membrane in this fold, resulting in no opportunity for additional fold formation. Accordingly, the other areas of the membrane, in particular the areas with the perforation slits, continue to lay smoothly against the introduction body element under the effect of hydrostatic pressure when the gas feed is switched off, and is therefore not subjected to folding.
This arrangement of having fold-forming material in the upper area, related to the installation position, makes use of the tendency already present towards uncontrolled fold formation in the upper half of the gasification device. In this situation, the hydrostatic pressure in the lower part is less than in the upper part, and the membrane is initially in contact at the bottom of the introduction body element when the gas feed is switched off.
Without the arrangement of the present invention, this process would not uniformly take place since flows in the waste water also play a part, and the forces resulting from this are superimposed on the hydrostatic pressure. It is then possible for folds to form beneath the crest, in other words the highest-lying line of the membrane, which has been demonstrated to be the case in practice.
According to a further embodiment, the arrangement can be formed by a longitudinal section of the membrane with an impressed camber radius which is smaller than the mean camber radius of the membrane measured in the fitted position on the introduction body element.
The hydrostatic forces, which take effect radially on the membrane when the gas feed is switched off, support the inclination towards fold formation of the more sharply cambered area of the membrane.
In another embodiment, the arrangement is provided by a longitudinal web, which projects over the circumference of the casing surface of the introduction body element.
The advantage of this design is it is possible to avoid preliminary deformation of the membrane. In addition, when the membrane is contracted, it is more cambered at one point than in the other areas. The same effect of the hydrostatic forces taking effect radially on the membrane is then exploited, which also leads to fold formation at the desired point with the pre-deformed membrane.
Several variants are possible with the design and arrangement of the longitudinal web; for example, the web can be an integral part of the introduction body element.
This requires a modification of the manufacturing mold in comparison with the conventional design. However, with subsequent manufacture, no relevant additional costs are incurred since the fitting of the membrane to the introduction body element can take place without change.
In addition, the longitudinal web may also be a separate component, which can be inserted between the membrane and the introduction body element.
With this design, it is possible to avoid modification of the manufacturing mold, although an additional working stage is required during assembly. It may also be necessary for the longitudinal web to be fixed to the introduction body element.
A further embodiment provides that the longitudinal web is secured on the side of the membrane which faces inwards. Therefore, it is possible to avoid modification of the manufacturing mold for the introduction body element. However, an additional working stage is still necessary.
The longitudinal web can also be arranged on the inwardly facing side of the membrane and be an integral part thereof.
In this embodiment, a modification to the mold for the manufacture of the membrane is necessary. However, no relevant additional costs are incurred in the subsequent installation.
In a further embodiment, the arrangement can be formed by a longitudinal area of the membrane with an impressed camber radius, which provides a camber directed inwardly with respect to the introduction body element.
In this case, the camber radius is smaller than the mean camber radius of the membrane when measured when mounted on the introduction body element.
When the gas feed is turned off, the hydrostatic forces taking effect radially on the membrane cause a fold formation in the pre-treated longitudinal section of the membrane. In this situation, the camber radius in the centre of this longitudinal section increases, while the laterally delimited areas form folds with the camber in the opposite direction.
Finally, it is possible for a longitudinal groove to be arranged in the introduction body element in the area of the longitudinal area of the membrane.
This longitudinal groove allows for an inward deflection of the increasing crest of the longitudinal area, under the influence of the hydrostatic pressure, in relation to the introduction body element.